THE DIGESTIVE SYSTEM

PART II

BY

MEDICAL PHYSIOLOGY

DEPARTMENT – FACULTY OF

MEDICINE – MINIA UNIVERSITY

DEFINITIONS

❑ Steatorrhea: means passing of greasy bulky pale

stools due to deficient bile (i.e. required for fat

digestion).

❑ Choleretic: it is a substance that increases the bile

secretion from the liver (e.g. bile salts, vagal

stimulation and secretin hormone).

❑ Cholagugue: it is a substance that helps the

evacuation of the gall bladder (e.g. Cholecystokinin

hormone (CKK).

JAUNDICE

It is a yellowish discoloration of the skin, sclera and mucous

membranes due to increased serum bilirubin level ≥ 2 mg/dl.

DEFINITION

JAUNDICE

TYPES AND CAUSES OF JAUNDICE

Points Haemolytic Jaundice Obstructive Jaundice Hepatocellular Jaundice

Cause

• Incompatible blood

transfusion.

• All causes of

haemolytic anaemia

(e.g. hereditary

spherocytosis, sickle cell

anaemia, poisons…

• Stones in the common

bile duct (CBD).

• Cancer head of

pancreas.

• Liver diseases (e.g.

hepatitis, cancer, liver

cirrhosis, bilharzias…

Blood

Anaemia

Bilirubin

Present

Haemobilirubin

Absent

Cholebilirubin

Absent

both

Urine Colour Normal Dark brown brown

Stool Colour Darker than normal Very pale pale

Gastrointestinal Motility

I. MASTICATION (CHEWING)

DEFINITION:

The chewing movements break the food into small pieces and

stimulate the secretion of saliva to make a suitable food bolus

ready for swallowing.

The act of chewing is partly voluntary and partly reflex in

nature.

Functions of Mastication:

1. Break down of food into small particles to increase the

surface area of food exposed to the action of digestive

enzymes.

2. It stimulates salivary, gastric pancreatic and bile secretions.

3. Reduce mechanical damage of GIT.

4. Development of satiety through stimulation of smell and

taste receptors.

I. MASTICATION (CHEWING)

Definition: It is the passage of food from mouth to stomach.

Stages (Phases) of Deglutition:

1. Buccal Stage:

In which food passes from mouth to pharynx. It is the voluntary

phase.

2. Pharyngeal Stage:

In which food passes from the pharynx to the esophagus. It is

involuntary.

This phase is sometimes called the Deglutition Reflex:

• Stimulus: touching the pharynx (swallowing receptors) by

food.

• Afferent: trigeminal nerve.

II. DEGLUTITION (SWALLOWING)

• Center: swallowing center in medulla oblongata.

• Efferent: glossopharyngeal and vagus nerve.

• Responses: as follows;

a. Protective Reflexes:

➢ Elevation of soft palate to close posterior nasal opening to

prevent regurgitation of food to nose.

➢ Elevation of the larynx to be covered by epiglottis to

prevent entry of food to the trachea.

➢Approximation of the vocal cords.

➢ Temporary apnea as swallowing center inhibits respiratory

center few seconds.

b. Pharyngeal Peristalsis:

Contraction of pharyngeal muscles and relaxation of

upper part of esophagus (upper esophageal sphincter)

to push the food bolus to the esophagus.

N.B.

In anesthesia, cough and swallowing center are

depressed and the protective reflexes don't occur so

secretion or vomitus may accumulate in pharynx and

enter the trachea → choking (i.e. inability to

breathe).

3. Esophageal stage:

• In which food passes from esophagus to stomach.

• It is involuntary.

Stages of Deglutition (i.e. Swallowing).

III. GASTRIC MOTILITY

Three types;

1. Receptive Relaxation:

• A reflex relaxation of stomach wall initiated by the movements of the

pharynx and esophagus and increases when food enters the stomach

especially in the proximal unit of the stomach (i.e. fundus and body).

• Function; it allows the stomach to accommodate up to 1.5 liters of

fluids without marked increase in intragastric pressure (i.e. plasticity).

2. Gastric Peristalsis:

• Peristaltic waves occur in the distal motor unit of the stomach (i.e.

antrum and pylorus) shortly after food intake.

• Function; it helps in mixing the food with gastric juice and pushing it to

the duodenum.

3. Hunger Contractions:

• Strong peristaltic contractions that occur in the body of the

stomach when it is empty for long time (e.g. prolonged

fasting).

Mechanism:

• Prolonged fasting ….. Causes hypoglycemia ….. Activates

the feeding center which in turn ….. Activates vagal nucleus

in medulla oblongata ….. Hunger contractions.

• That is why insulin stimulates hunger contractions while

glucose inhibits it

VOMITING

• Definition: It is a reflex evacuation of upper GIT contents

(mainly stomach) through the mouth.

• It is preceded by salivation, sweating, rapid heart rate and

sensation of nausea.

Causes of Vomiting:

1.Reflex vomiting; as a result of smell, taste, vision, severe

pain, motion sickness …

2.Central vomiting; as a result of hypoxia, head trauma,

drugs, ketosis …

Effects of Persistent Vomiting:

1.Dehydration (i.e. water loss).

2.Alkalosis (due to loss of gastric HCl).

3.Potassium depletion.

4.Tetany.

Treatment of Vomiting:

1.Treat the cause.

2.Anti-emetic drugs.

3.Correct the complications by giving I.V. fluids, electrolytes

(K+) and acidifying salts.

IV. INTESTINAL MOTILITY

Two types;

Points Peristaltic movementSegmenting

movement

Definition

A wave of deep

circular contraction

followed by a wave

of relaxation.

Ring like contractions;

appear at regular

interval along the

intestine.

Mechanism Neurogenic. Myogenic.

Function Abolished by cocaine.Not abolished by

cocaine.

Effect of local

anaesthetic

(e.g. cocaine)

Push intestinal contents.Help digestion and

absorption.

DEFECATION

Definition:

It is evacuation of the colon through the anus.

Mechanism:

• It is pure automatic reflex in animals and infants of

human.

• It is under voluntary control in adult human.

Defecation Reflex:

• Stimulus: distension of rectum by stools.

• Receptors: stretch receptors in rectal wall.

• Afferent: pelvic nerve.

• Center: defecation center in spinal cord (S 2, 3 and 4).

• Efferent: pelvic nerve.

• Response; either:

➢ If conditions are favorable → contraction of the rectal wall

and relaxation of internal anal sphincter → Defection.

➢ If the conditions are not favorable → the cerebral cortex

sends inhibitory impulses to the defecation center in the

spinal cord → Temporary inhibition of defecation.

DISORDERS OF DEFECATION

Definition: In normal persons, the frequency of defecation

ranges from several times per day to once every 2-3

days.

Constipation is an abnormal condition characterized by

prolonged interval between defecation associated with

difficulty in performing the act.

Causes of Constipation:

1. Repeated voluntary inhibition of the act. This leads to

overstay of stool in the colon, resulting in more water

reabsorption from it.

1. CONSTIPATION

2. Decreased motility of the colon as occurs in

prolonged ingestion of food with little residue, thus

the bulk of stool becomes ineffective to initiate

defecation reflex, e.g. old age, lack of muscular

exercise, debilitating disease and emotional stress

(i.e. spastic colon).

Effects of Constipation:

1. Abdominal discomfort and distension.

2. Nausea and anorexia.

3. Headache.

2. DIARRHEA

• DEFINITION: Abnormal condition characterized by

increased frequency of defecation with excretion of

watery stool.

Causes of Diarrhea:

• Hypermotility of the small intestine due to Intestinal

infection either viral or bacterial

Effects of Diarrhea:

Diarrhea is much more dangerous than constipation

especially in infants, because it may lead to loss of water

(dehydration), electrolytes and bicarbonate (i.e. metabolic

acidosis).

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